1. Field of the Invention
The present invention relates generally to a motor-driven power steering control system for a motor vehicle. More specifically, the invention is concerned with a control apparatus for controlling the operation of the motor-driven power steering system which incorporates as a driving source an electric motor energized with a current supplied from an onboard battery of the motor vehicle for generating an assist torque which is applied to an output shaft of the steering system for aiding or assisting a driver in his or her manipulation of a steering wheel.
2. Description of Related Art
As the motor-driven power steering system known heretofore, there may be mentioned, for example, a system disclosed in Japanese Unexamined Patent Application Publication No 94227/1995 under the title "MOTOR-DRIVEN POWER STEERING SYSTEM". In this known motor-driven power steering system, an assist torque is applied to an output shaft operatively coupled to a steering column of the steering system, wherein a magnitude of the assist torque as well as the direction thereof is controlled in accordance with a steering toque applied by a driver to the steering system through the steering wheel of the motor vehicle. To this end, the conventional motor-driven power steering system is provided with a torque sensor for detecting the steering torque applied by the driver. More specifically, a torsion bar is interposed between an input shaft and the output shaft of the steering column, wherein the steering torque applied to the steering wheel is detected by detecting a magnitude of the torsion of the torsion bar as well as a direction thereof.
The conventional motor-driven power steering system further includes a control means for arithmetically determining an assist torque to be applied by an electric motor to the output shaft of the steering column or a steering rack on the basis of the torque detection value derived from the output of the torque sensor so that the manipulation of the steering wheel by the driver becomes optimal with the aid of the assist torque. In such a motor-driven power steering system, the electric power for driving the electric motor dedicated for the generation of the assist torque is usually supplied from an onboard battery mounted on the motor vehicle. Accordingly, with a view to protecting the battery from excessive power consumption which may eventually lead to a so-called dead battery state, operation of the motor is so controlled as to be stopped when engine stall takes place in the course of driving the motor vehicle or when an ignition key of the internal combustion engine is turned off.
In the conventional motor-driven power steering system implemented in the structure mentioned above, there arises such problem that when the engine of the motor vehicle is stopped due to the stall in the course of the steering operation or the opening of the key switch, the assist torque applied to the steering system disappears abruptly because operation of the electric motor and hence generation of the assist torque are so controlled as to be stopped concurrently with the stoppage of operation of the electric motor. As a consequence, there may occur such undesirable situation that the steering wheel is forced to rotate rapidly or abruptly in the direction opposite to the steering direction under the elastic restitution of the torsion bar and the tiers, which provides a great obstacle to the smooth steering operation, remarkably degrading the comfort in driving the motor vehicle. In an extreme case, the driver may have a problem in coping with the abrupt behavior of the steering operation.